Orthogonal frequency division multiplexing (ofdm) communications device and method that incorporates low papr preamble and variable number of ofdm subcarriers

ABSTRACT

A communications device includes communications data and a training sequence corresponding to a preamble. A modulation and mapping circuit modulates the communications data and training sequence into a plurality of multiple subcarriers that are orthogonal to each other to form an OFDM communication signal having modulated subcarriers carrying the communications data forming a data payload and modulated subcarriers forming the preamble. A circuit turns OFF and ON selected subcarriers based on an encryption algorithm to increase transmit power and signal-to-noise ratio and reduce the Inter-Carrier Interference (ICI). The modulation and mapping circuit applies a quadratic modulation to the subcarriers carrying the training sequences to produce a low peak-to-average power ratio (PAPR) preamble with a PAPR value of approximately 2.6 decibels (dB).

FIELD OF THE INVENTION

The present invention relates to communications systems, and moreparticularly, this invention relates to multiple carrier communicationsystems, including but not limited to, Orthogonal Frequency DivisionMultiplexing (OFDM) communications systems.

BACKGROUND OF THE INVENTION

In OFDM communications systems the frequencies and modulation of afrequency-division multiplexing (FDM) communications signal are arrangedorthogonal with each other to eliminate interference between signals oneach frequency. In this system, low-rate modulations with relativelylong symbols compared to the channel time characteristics are lesssensitive to multipath propagation issues. OFDM thus transmits a numberof low symbol-rate data streams on separate narrow frequency subbandsusing multiple frequencies simultaneously instead of transmitting asingle, high symbol-rate data stream on one wide frequency band on asingle frequency. These multiple subbands have the advantage that thechannel propagation effects are generally more constant over a givensubband than over the entire channel as a whole. A classicalIn-phase/Quadrature (I/Q) modulation can be transmitted over individualsubbands. Also, OFDM is typically used in conjunction with a ForwardError Correction scheme, which in this instance, is sometimes termedCoded Orthogonal FDM or COFDM.

As known to those skilled in the art, an OFDM signal can be consideredthe sum of a number of orthogonal subcarrier signals, with baseband dataon each individual subcarrier independently modulated, for example, byQuadrature Amplitude Modulation (QAM) or Phase-Shift Keying (PSK). Thisbaseband signal can also modulate a main RF carrier.

These types of multi-carrier waveforms for digital communicationsrequire a summation of multiple frequency-spaced single-carriers priorto transmission through a Power Amplifier (PA). OFDM systems typicallyuse a hardware efficient IFFT to modulate each individual subcarrierwith a QAM symbol and sum the modulated complex exponentials together toproduce a single time-domain waveform, which has a very largePeak-to-Average Power (PAPR) ratio. As a result, the average power intothe PA must be “backed-off” to avoid clipping of the time-domain signalpeaks. This clipping significantly increases the in-band noise (IBN) andthe out-of-band noise (OBN) and adversely increases the Bit Error Rate(BER) and the Adjacent Channel Interference (ACI), respectively.

Terrestrial wireless communications systems usually encounter multipathfading channels, which are typically modeled using a Rician direct pathand several additional Rayleigh paths. Most standard OFDM systems use aset of known training symbols to estimate the wireless channel'sfrequency response. These training symbols are transmitted during thebeginning of a packet and form the preamble. Occasionally trainingsymbols are transmitted during the middle of a packet and form amidamble. This preamble provides to the receiver known amplitude andphase references at each of the subcarrier frequencies. The preambleX(f) is stored in memory and known in advance at the receiver. Thetransmitter sends x(t) [the inverse Fourier Transform of X(f)] over thewireless channel to the receiver. During its transmission to thereceiver, the transmitted preamble representation x(t) is convolved withthe channel's time response h(t) to produce y(t) at the receiver. Thereceiver uses Y(f) [the Fourier transform of y(t)] to calculateH(f)=Y(f)/X(f), the channel frequency response. In order to compensatefor the channel response at the receiver, incoming data symbols aremultiplied by H⁻¹(f), which is equivalent to the convolution in thetime-domain with the inverse channel response.

Conventional OFDM systems use a preamble designed for a specific numberof subcarriers. Each preamble has a low PAPR by design. Optimizing apreamble's PAPR for a certain number of subcarriers N does not hold whenthe number of subcarriers changes. Therefore, if the number ofsubcarriers in the system changes, a new low PAPR preamble based on thechanged (but fixed) number of subcarriers is required. Thus, currentOFDM systems design and implement special preambles for each fixednumber of subcarriers. Typically, OFDM implementations that allow adifferent number of subcarriers have a small number of combinations ofsubcarrier sizes and preambles to select from based on which fixednumber of subcarriers will be transmitted. In practical OFDM systems,the PAPR may be reduced using one or a combination of severaltechniques.

Some OFDM systems use nonlinear signal distortion such as hard clipping,soft clipping, companding, or predistortion techniques. These nonlineardistortion techniques are implemented in fairly simple circuits. They donot work well, however, in cases where the OFDM subcarriers aremodulated with higher order modulation schemes. In such situations, theEuclidian distance between the symbols is relatively small and theadditional noise introduced by the PAPR reduction causes significantperformance degradation.

A second group of OFDM systems reduce PAPR using various codingtechniques, which are typically distortionless. The PAPR reduction ismost commonly achieved by eliminating symbols having a large PAPR. Toobtain an appreciable level of PAPR reduction, however, high redundancycodes are used and as a result, the overall transmission efficiency isreduced.

A third group of OFDM systems minimize PAPR based on OFDM symbolscrambling and selecting a sequence that produces minimum PAPR. Thesepre-scrambling techniques achieve good PAPR reduction, but typicallyrequire multiple FFT transforms and higher processing power. An exampleof OFDN systems using some type of scrambling are the OFDMcommunications systems and methods disclosed in commonly assigned U.S.patent application Ser. Nos. 11/464,877; 11/464,857; 11/464,854;11/464,861; and 11/464,868, filed on Aug. 16, 2006, the disclosureswhich are hereby incorporated by reference in their entirety. Furtherenhancements to those systems are found in commonly assigned U.S. patentapplication Ser. Nos. 12/060,283; 12/060,311; and 12/060,292, filed Apr.1, 2008, the disclosures which are hereby incorporated by reference intheir entirety

An OFDM waveform has a very large Peak to Average Power Ratio (PAPR). Toavoid clipping and nonlinear distortion, the OFDN transmitter's PowerAmplifier (PA) needs to be operated a significantly lower power levelthan its peak rating (i.e., “backed off”). This paper proposes a novellow PAPR preamble for channel estimation. This solution yields apreamble with a typical PAPR of 2.6 dB, regardless of the number ofsubcarriers. The subcarriers are evenly spaced and equal amplitude,ideal for channel sounding. The result is a preamble construction thatoperates over a variable number of subcarriers, improves SNR, and iseasy to implement. The theoretical basis for the technique is presented,and a particular implementation of the technique in hardware isdiscussed. Both simulations and measurements demonstrate verysignificant benefits of the technique in long-range wirelesscommunications applications that use an OFDM waveform for radiocommunications.

SUMMARY OF THE INVENTION

A communications device includes communications data and a trainingsequence corresponding to a preamble. A modulation and mapping circuitmodulates the communications data and training sequence into a pluralityof multiple subcarriers that are orthogonal to each other to form anOFDM communication signal having modulated subcarriers carrying thecommunications data forming a data payload and modulated subcarriersforming the preamble. A circuit turns OFF and ON selected subcarriersbased on an encryption algorithm to increase transmit power andsignal-to-noise ratio per subcarrier and reduce the IntercarrierInterference (ICI). The modulation and mapping circuit applies aquadratic modulation to the subcarriers carrying the training sequencesto produce a low peak-to-average power ratio (PAPR) preamble with abouta PAPR value of 2.6 decibels (dB).

A circuit spreads the selected ON subcarriers over the frequency domainto reduce average power for enhanced Low Probability of Interception/LowProbability of Detection (LPI/LPD) and allows for more transmit powerwithin a spectral mask and reduces adverse effects of Frequency SelectedFading. The pseudo-random signal generator is operative with themodulation and mapping circuit and generates pseudo-random signals basedon an encryption algorithm for frequency hopping each subcarrier at anOFDM symbol rate. This low PAPR preamble can be independent of thenumber of subcarriers forming the OFDM communications signal.

The modulation and mapping circuit is also operative for settingquadratic phases of sinusoids forming the subcarriers carrying thetraining sequences and setting the phases to a specific angle. Thepreamble can include a long sync sequence for channel and fine frequencyoffset estimation having a guard interval (GI). The subcarriers thatcarry the training sequences can have an equal amplitude.

In yet another aspect, the guard interval is split from the long syncsequence and its inverse is processed into values that represent the lowPAPR preamble as plus or minus one (±1) values in a real or imaginarycomponent as adds and subtracts for channel estimation.

A method aspect is also set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIGS. 1A and 1B are prior art, high-level block diagrams showingrespective transmitter and receiver circuits for an IEEE 802.11a OFDMmodem connected through a radio transmission channel.

FIGS. 2A-2C are spectrum graphs representing a) a single carrier signal;b) a Frequency Division Multiplexing (FDM) signal; and c) an OrthogonalFrequency Division Multiplexing (OFDM) signal.

FIG. 3A is a graph showing a three-dimensional representation of aconventional OFDM signal.

FIG. 3B is a spectrogram showing a power distribution for an example ofa conventional OFDM signal such as shown in FIG. 3A.

FIG. 3C is a graph showing a two-dimensional representation of a typicalOFDM spectrum such as shown in FIG. 3A.

FIG. 3D is graph for a 64-QAM constellation of a conventional OFDMsignal such as is shown in FIG. 3A.

FIGS. 4A-4D are spectral density graphs with each graph showing an OFDMspectrum, with one graph showing 52 subcarriers ON, and comparing thisgraph with a graph of spectrum in which a respective 26 subcarriers areON, another graph showing 13 subcarriers are ON, and another graphshowing 6 subcarriers are ON, producing a reduced Inter-CarrierInterference (ICI), while FIG. 4E is an explanation and equationrepresenting total transmit power from a spectral density function.

FIG. 5 is a graph showing a three-dimensional spectrogram (power vs.frequency vs. time) of Symbol-Based and Frequency Randomized subcarriersfor a frequency hopping OFDM signal that could incorporate the low PAPRpreamble in accordance with a non-limiting example of the presentinvention, and also showing a conventional single carrier signaloverlaid for comparison.

FIG. 6 is a high-level block diagram of a transmitter that can be usedfor generating a frequency hopping OFDM signal that could be modifiedand incorporate the low PAPR preamble in accordance with a non-limitingexample of the present invention.

FIG. 7 is a high-level block diagram of a receiver that can be used forreceiving and processing a transmitted frequency hopping OFDM signalthat incorporates the low PAPR preamble and showing the demapping anddemodulation circuit and including a channel estimate circuit forestimating channel response using the low PAPR preamble in accordancewith a non-limiting example of the present invention.

FIG. 8 is a graphical and frequency representation of a typical OFDMpreamble as established by IEEE that can be modified as a low PAPRpreamble in accordance with the non-limiting example of the presentinvention.

FIG. 9 is a graph showing the FFT of 64 subcarriers in an OFDMcommunications system.

FIG. 10 is a graph showing the complex envelope of 64 carriers atzero-phase in an OFDM communications system.

FIG. 11 is a graph showing the complex envelope of 64 carriers atzero-phase in dB (decibels) in an OFDM communications system.

FIG. 12 is a graph showing the quadratic phases in an OFDMcommunications system such as for applying to the low PAPR preamble inaccordance with non-limiting examples.

FIG. 13 is a graph showing the zero-phase versus the quadratic-phasePAPR (dB) in an OFDM communications system and showing an optimumquadrature phase for minimal PAPR.

FIG. 14 is a graph showing the PAPR of AM modulated quadratic phasesignals in an OFDM communications system.

FIG. 15 are graphs for the real and imaginary signal representations ofa low PAPR preamble frequency-domain representation in accordance with anon-limiting example of the present invention.

FIG. 16 are graphs for real and imaginary values for the low PAPRpreamble long Sync 1 reference in accordance with a non-limiting exampleof the present invention.

FIG. 17 are graphs for the real and imaginary values for the low PAPRpreamble long Sync 0 reference in accordance with a non-limiting exampleof the present invention.

FIG. 18 is another block diagram of an OFDM receiver similar to thatshown in FIG. 7 that can be used for processing the low PAPR preamble inaccordance with a non-limiting example of the present invention andusing a channel estimate circuit within the receiver circuitry.

FIG. 18A is a block diagram showing an OFDM transmitter operative fortransmitting an OFDM signal with low PAPR preamble to a filter fordetermining the filter response in accordance with a non-limitingexample.

FIG. 19 is a block diagram of an example of a communications system withvarious communications devices that can be used and modified inaccordance with a non-limiting example of the present invention

FIG. 20 is a high-level block diagram showing basic components of atransmitter and receiver used in communications devices that can be usedand modified in accordance with a non-limiting example of the presentinvention.

FIG. 21 is a perspective view of a portable wireless communicationsdevice as a handheld radio that could incorporate the communicationssystem and radio as a communications device modified to work inaccordance with a non-limiting example of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

In accordance with a non-limiting example, the low PAPR preambleprovides greater transmit power during the preamble and provides forbetter channel equalization. The PAPR of the preamble is typically 2.6dB regardless of the number of subcarriers. A greater signal-to-noiseratio can be allowed for channel equalization, allowing a lower biterror rate (BER). Typically, in OFDM systems, a higher signal-to-noiseratio is required for detection because of the robust turbo/LDPC(low-density parity-check) forward error correction (FEC) coding schemesas compared to data symbol decoding. The system and method provides forhigh signal-to-noise ratio for detection. It also permits a lower PAPRthan most existing preamble techniques used with OFDM systems andprovides good correlation properties for receiver detection. The lowPAPR preamble is operable in OFDM systems having a variable number ofsubcarriers and can be implemented in efficient hardware implementationwith IFFT/FFT circuits.

The low PAPR preamble, in accordance with a non-limiting example,requires less back-off from the power amplifier's P1 dB point, allowinghigher transmit power. When subcarriers are evenly spaced at equalamplitude, the OFDM communications system incorporating the low PAPRpreamble is ideal for channel sounding.

This use of the low PAPR preamble, in accordance with non-limitingaspects, is advantageous for both the commercial market and governmentmarket that demands higher data rates and increased spectral efficiencywith better bandwidth use. This low PAPR preamble allows increased datarates and spectral efficiency for OFDM systems even when the number ofsubcarriers is increased. It overcomes the drawbacks typicallyassociated with OFDM systems that use a large number of subcarriers thatlead to high PAPR, create a demand for larger power amplifiers, andincrease cost, size, weight and power of the associated devices. Thesystem and method allows a reduced PAPR preamble, independent of thenumber of subcarriers, and provides optimal channel estimation and goodcorrelation properties for receiver detection. It is easily implementedvia an IFFT/FFT. The lower PAPR preamble allows increased transmit powerusing power amplifiers with lower cost, size, weight and power.

FIGS. 1A and 1B are high-level block diagrams showing basic circuitcomponents of an IEEE 802.11a OFDM modem, and showing the transmittercircuit 30 in FIG. 1A and the receiver circuit 32 in FIG. 1B. Thetransmitter circuit 30 (also termed “transmitter” for clarity) transmitsan OFDM signal as shown in FIG. 2C. By comparison, FIG. 2A shows thespectrum of a single carrier signal and FIG. 2B shows in comparison tothe single carrier signal of FIG. 2A, the spectrum of a classicalFrequency Division Multiplexing (FDM) signal. FIG. 2C shows the spectrumof an OFDM signal.

The drawings in FIG. 2A-2C show that OFDM is based on afrequency-division multiplexing (FDM) system where each frequencychannel is modulated. The frequencies and modulation of an FDM systemare now orthogonal to each other to eliminate interference betweenchannels. Because low-rate modulations with relatively long symbolscompared to the channel time characteristics are less sensitive tomultipath, an OFDM communications system allows a number of low-ratesymbol streams to be transmitted simultaneously on multiple carriersrather than having one high-rate symbol stream transmitted on a singlecarrier. Thus, the frequency spectrum in an OFDM communications systemis divided into multiple low-bandwidth subbands. Since each subbandcovers a relatively narrow section of the frequency spectrum, channelpropagation effects are more constant or “flat” over a given subbandcompared to channel variations over the entire occupied spectrum. Anytype of in-phase and quadrature (I/Q) modulation can be used to modulateany subcarrier, for example, Binary Phase Shift Keying (BPSK),Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation(QAM), or any of the numerous and different derivations of thesemodulation schemes. Different signal processing techniques, for example,channel coding, power allocation, adaptive modulation encoding, andsimilar schemes can be applied to one or more subbands. Multi-userallocation is also possible for example using time, coding, or frequencyseparation.

In an OFDM communications system using a transmitter and receiver suchas shown in FIGS. 1A and 1B, one transmitter will transmit a signal ondozens or thousands of different orthogonal frequencies that areindependent with respect to the relative amplitude and phaserelationship between the frequencies. Each subcarrier signal typicallywill have space for only a single narrowband signal because the signalsare closely spaced and it is important to prevent signals on adjacentsubcarriers from interfering with each other. In an OFDM system, thesymbols on each subcarrier are constructed such that energy from theirfrequency components are zero at the center of every other subcarrier,enabling a higher spectral efficiency for OFDM symbols than is possiblein classic FDM.

The OFDM system as shown in FIGS. 1A and 1B includes channel coding as aForward Error Correction (FEC) technique, using a Forward ErrorCorrection encoder to create a coded orthogonal FDM (COFDM) signal.Channel-State Information (CSI) techniques can also be employed,including continuous wave (CW) interferer and/or selective channelsystems.

An OFDM signal is typically the sum of each of the orthogonalsubcarriers. Baseband data is independently modulated onto each of theorthogonal subcarriers using some type of modulation, such as QuadratureAmplitude Modulation (QAM) or Phase Shift Keying (PSK) schemes. Becausethe spectrum of each subcarrier overlaps, it can be considerably widerthan if no overlap were allowed. Thus, OFDM provides high spectrumefficiency. Because each subcarrier operates at a low symbol rate, theduration of each symbol in the subcarrier is long. (For clarity, “symbolrate” is equal to the inverse of “symbol duration.”) By using ForwardError Correction (FEC) equalization and modulation, there can be anenhanced resistance against a) link dispersion, b) slowly changing phasedistortion and fading, c) frequency response nulls, d) constantinterference, and e) burst noise. Further, the use of a Guard Interval(GI) or cyclic prefix provides enhanced resistance against multipath inthe transmission channel.

Typically, in an OFDM communications system, a subcarrier and somewhatrectangular pulse is subjected to an Inverse Discrete Fourier Transform(IDFT) using an Inverse Fast Fourier Transform (IFFT) circuit within thetransmitter. At a receiver, a Fast Fourier Transform (FFT) circuitreverses this operation. The rectangular pulse shape results in aSin(x)/x spectrum in the subcarriers.

The spacing of subcarriers can be chosen such that the receivedsubcarriers can cause zero or acceptably low Inter-Carrier Interference(ICI) when the receiver and transmitter are synchronized. Typically,OFDM communications systems split the available bandwidth into manynarrow-band subbands from as little as a few dozen to as many as eightthousand to ten thousand. Unlike the communications system providingmultiple channels using classical FDM as in FIG. 2B, the subcarriers foreach subband in OFDM are orthogonal to each other and have close spacingand little overhead, In an OFDM communications system, there is alsolittle overhead associated with any switching that may occur betweenusers as in a Time Division Multiplexing Access (TDMA) communicationssystem. Usually, the orthogonality of subcarriers in an OFDMcommunications system allows each carrier to have an integer number ofcycles over a symbol period. As a result, the spectrum of a subcarrierhas a null at the center frequency of its adjacent subcarriers.

In an OFDM communications system, the spectrum required for transmittingdata is chosen based on the input data and a desired modulation schemeto be used with each carrier that is assigned the data to transmit. Anyamplitude and phase of the carrier is calculated based on themodulation, for example, BPSK, QPSK or QAM as noted before. Any requiredspectrum is converted using an IFFT circuit to ensure carrier signalsare orthogonal.

It should be understood that a FFT circuit transforms a cyclic timedomain signal to an equivalent frequency spectrum by finding anequivalent waveform that is generated as a sum of orthogonal sinusoidalcomponents. The frequency spectrum of the time domain signal is usuallyrepresented by the amplitude and phase sinusoidal components. The IFFTcircuit performs the reverse process and transforms the spectrum of theamplitude and phase into a time domain signal. For example, an IFFTcircuit can convert a set of complex data points into a time domainsignal of the same number of points. Each complex input point willresult in an integral number of sinusoid and cosinusoid cyclesrepresented by the same number of points as were input to the IFFT. Eachsinusoid known as the in-phase component, and cosinusoid known as thequadrature component, will be orthogonal to all other componentsgenerated by the IFFT. Thus, orthogonal carriers can be generated bysetting an amplitude and phase for each frequency point representing adesired subcarrier frequency and performing the IFFT,

It should be understood that a guard interval (GI), also termed a cyclicprefix, often is added to an OFDM symbol The guard interval reduces theeffects of the wireless channel on Inter-Symbol Interference (ISI) andcontains redundant transmission information. For example, in the IEEE802.11a standard, if a carrier spacing is 312.5 KHz, and the FourierTransforms are performed over 3.2 microseconds, then a 0.8 microsecondguard interval can be applied for ISI rejection. This guard intervalcould be the last T_(g) seconds of an active symbol period that isprefixed to an OFDM symbol, making it a cyclic prefix. It is kept shortfor a fraction of “T,” corresponding to the total length of the activesymbol, yet longer than the channel impulse response. This helps reducethe ISI and Inter-Carrier Interference (ICI) and maintain subcarrierorthogonality. In this example, a time waveform appears periodic to thereceiver over the duration of the FFT.

To reduce ICI, the OFDM symbol can be cyclically extended in the guardtime to ensure that delayed replicas of the OFDM symbol can have aninteger number of cycles within the FFT interval, as long as the delayis smaller than the guard time. As a result, multipath signals withdelays smaller than the guard time would not produce ICI.

Multipath interference is typically caused when multiple copies of thetransmitted signal arrive at the receiver at different times. It shouldbe understood that an OFDM communications system reduces the effect ofmultipath interference by providing the ability to add signal redundancyin both frequency and time by the use of various coding algorithms. Forexample, with the IEEE 802.11a standard using OFDM, 48 carriers can betransmitted simultaneously. The coding gain can be provided using aone-half (½) convolutional encoder at the transmitter and later aViterbi decoder at the receiver. Data bits can be interleaved acrossmultiple symbols and carriers. Lost data often is recoverable because ofinterleaving across the frequency and time space.

Increasing the data rate requires an increase in the symbol rate for afixed number of carriers, fixed modulation scheme and fixed sample rate.For a single carrier system, complex equalizers and adaptive filters arerequired at the receiver to compensate for the magnitude and timedistortions caused by the channel. The accuracy and dynamic rangerequired of such equalizers and filters increases markedly as symboltimes are decreased. However, in an OFDM system, for example, when 48subcarriers are transmitted simultaneously, the symbol rate iseffectively reduced by 48 times, significantly reducing the requirementsof channel equalizers and filters. The reduced symbol rate of an OFDMsystem enables a robust communication link, resistant to ISI.

It should be understood that an OFDM receiver receives a sum of thedifferent signals as subcarriers. The addition of a guard interval canfurther enhance performance in an OFDM system by ensuring that no symboltransitions occur during each received symbol time. For example, if anOFDM subcarrier is BPSK modulated, there would be a 180 degree phasejump at symbol boundaries. By choosing a guard interval that is longerthan the largest expected time difference between the first and lastmultipath signals, such phase transitions can occur only during theguard time, meaning there are no phase transitions during the FFTinterval. If the phase transitions of a delayed path occur within theFFT interval of the receiver, then the summation of the subcarriers ofthe first path with the phase modulated waves of the delayed path wouldno longer produce a set of orthogonal subcarriers, resulting in acertain level of interference.

FIG. 1A illustrates a high-level block diagram of the prior arttransmitter 30 for an IEEE 802.11a OFDM modem such as generallydescribed above, and includes a source of communications data 33 and aForward Error Correction (FEC) Coder circuit 34 that receives a signalrepresenting the communications data 33, and encodes the signal with aforward error correction code as described above. The signal passes toan interleaving and mapping circuit 36 for interleaving and frequencymapping the communications data. An IFFT circuit 38 receives theinterleaved and frequency mapped signal and creates multiple time domaincarriers summed in a single in-phase/quadrature time domain sequenceknown as a symbol. A guard interval circuit 40 adds the cyclic prefixguard interval. A symbol wave shaping circuit 42, for example a raisedcosine filter, shapes the symbol waveform to limit its spectral content.Afterward, an In-phase/Quadrature (I/Q) modulator 44 processes thebaseband I/Q signal, producing I/Q modulation, and also receiving anintermediate frequency (IF) signal from LO signal generator 46. Signalup-conversion to the final transmit carrier frequency occurs at mixer48, which receives a local oscillator (LO) signal generated by LO signalgenerator 50. Afterward, the signal is amplified by a High PowerAmplifier (HPA) 52, and the OFDM signal is transmitted through anantenna 54 on its carrier wave into the RF channel 31. Various stages offrequency filtering, for example between the I/Q Modulator 44 and mixer48, and between the mixer 48 and HPA 52, and at the output of the HPA 52are not shown in the block diagram but typically used.

FIG. 1B shows a high-level block diagram of the prior art receivercircuit 32 used in the exemplary IEEE 802.11a OFDM modem. The antenna 60receives the OFDM signal from the RF Channel 31 on the carrier wave. Itis amplified within a low noise amplifier (LNA) 62. Signaldown-conversion occurs within a mixer 64, which also receives a localoscillator signal generated by an LO signal generator 66. An AutomaticGain Control (AGC) amplifier 68 provides automatic gain control to thedown-converted signal to ensure the appropriate signal level is appliedto the subsequent receiver circuitry. The AGC circuit uses a feedbacktechnique and is well known to those skilled in the art. In-phase andquadrature signal detection occurs within an I/Q Detect circuit 70,which also receives a local oscillator signal generated from a LO signalgenerator 72, which is also operative with an Automatic FrequencyControl (AFC) clock recovery circuit 74, as illustrated. The AFC circuitadjusts the local oscillator 72 frequency to keep the I/Q detector tunedappropriately. The I/Q Detect circuit 70, AFC clock Recovery circuit 74,and LO signal generator 72 form a feedback loop as illustrated and knownto those skilled in the art. The guard interval is removed within aRemove GI circuit 76. The Fast Fourier Transform (FFT) is applied on thesubcarriers as a reverse of the IFFT within an FFT circuit 78. Demappingand deinterleaving occur within a Demapping and Deinterleaving circuit80. Forward error correction decoding occurs within an FEC decoder 82,which finishes the signal processing and recovers the original data asreceived communications data 83. It is thus evident that the function ofthe receiver circuit 32 as shown in FIG. 1B operates in a mannerfunctionally the reverse of the transmitter circuit 30 shown in FIG. 1A.

As discussed above, OFDM communications systems can use FEC techniquesand known interleaving and mapping techniques before IFFT processing asshown in FIG. 1A, and demapping and deinterleaving techniques followedby FEC decoding after FFT processing as shown in FIG. 1B.

FIGS. 3A-3D are graphs showing different representations of aconventional OFDM signal, such as produced by the prior art OFDM modemtransmitter 30 shown in FIG. 1A.

FIG. 3A is a graph showing a three-dimensional representation of theOFDM signal with the frequency along one axis, time in seconds alonganother axis, and the “magnitude” or power on the vertical axis, forminga graph that indicates a magnitude vs. frequency vs. timerepresentation. It is evident from FIG. 3A that the OFDM signal can bedetected in the frequency domain. FIG. 3B is a graph showing aspectrogram or power distribution of the OFDM signal shown in FIG. 3A.FIG. 3C is a graph representing a two-dimensional OFDM spectrum of thethree-dimensional OFDM signal shown in FIG. 3A. FIG. 3D shows a 64-QAMconstellation for the OFDM signal shown in FIG. 3A. These graphstogether depict the power distributed over multiple subcarriers.

FIGS. 4A-4D are graphs showing a representation of the OFDM signalspectrum with different frequency subcarriers turned ON and OFF. In theupper left graph (FIG. 4A), the OFDM signal spectrum shows all 52carriers turned ON, indicating in this non-limiting example an IEEE802.11a standard using 52 carriers. In the upper right (FIG. 4B) 26subcarriers are ON, showing the transmit power per subcarrier having athree decibel increase over the 52 carrier case shown in FIG. 4A, due to26 carriers (subcarriers) being turned OFF. It should be understood thatthe total transmit power is equal to the area under the curve of thepower spectral density function. The lower left and lower right graphs(FIGS. 4C and 4D) show thirteen subcarriers and six subcarriers turnedON respectively. There is a transmit power increase of 6 decibels (6 dB)per subcarrier when 13 subcarriers are turned ON (FIG. 4C) and a 9decibel (9 dB) increase in transmit power per subcarrier when 6subcarriers are turned ON (FIG. 4D). Because the peak power is 6 dBhigher when the 13 subcarriers are turned ON, the distance in which thesignal will be useable (for a free-space channel) will be doubled.Further increases in range may be realized as power per subcarrier isincreased. FIG. 4D illustrates reduced Inter-Carrier Interference (ICI)due to the wide spacing of the subcarriers. FIG. 4E is an explanationand equation representing total transmit power from a spectral densityfunction.

It should be understood that OFDM coded transmissions may be easilydetected and received by unintended recipients by detection of datasubcarriers and pilot tones. The addition of multiple sine waves orcarriers with random amplitudes and phases to the waveform will cause itto approach a Gaussian distribution due to the central limit theorem. Asignal having a Gaussian random distribution inherently has an enhancedLow Probability of Interception (LPI) and Low Probability of Detection(LPD) because it appears similar to additive white Gaussian noise (AWGN)at a receiver.

In accordance with a non-limiting example, a modified transmitter for aflexible OFDM communications system as explained in detail below uses anIFFT to create multiple subcarriers located at specific frequencies.Only a small subset of the possible carriers need to be used at any onetime to enhance power and reduce ICI. Subcarrier center frequencies canbe changed at OFDM symbol times according to an encryption algorithm.Such an algorithm can generate a pseudo-random frequency hoppingsequence and frequency hopping subcarriers in accordance with anon-limiting example. Thus, fast-frequency hopping can change thesubcarriers' frequency for each OFDM symbol, and provide a one thousand(1,000) times faster frequency hopping than the Bluetooth standard, andten times its data rate. Additional benefits can include a reduced ICI,a reduced ISI, and reduced transmitter overhead from the guard interval.The system, apparatus and method in accordance with a non-limitingexample allows a symbol-based randomization for the OFDM signal.

A Walsh transform can be applied to spread subcarriers over thefrequency domain, in contrast with spreading over the time domain aswith conventional CDMA systems. Applying a Walsh transform to any subsetof subcarriers and then further processing using an IFFT circuit canreduce average power per frequency bin for enhanced LPI/LPD. Variousaspects of the communications system can be readily varied for improvedperformance. With fewer subcarriers as compared to the IFFT size and thespreading sequence length, more processing gain may be realized fromfrequency domain spreading. Furthermore, LPI/LPD and Anti-Jamming (AJ)performance can be enhanced, and there can be higher SNR per subcarrier.Increasing the sample rate also increases the bandwidth, data rate, andimproves the LPI/LPD/AJ performance.

FIG. 5 is a graph representing a three-dimensional spectrogram of thesymbol-based, frequency randomized subcarriers and showing a comparisonin log scale with a magnitude vs. frequency vs. time representation 501.A conventional single frequency carrier signal 502 is overlaid forcomparison and illustrated as a single carrier toward the lowerfrequency end of the band. This single carrier signal acts similarly toa jammer or interferer. The reduced Inter-Carrier Interference (ICI) isshown by an increased frequency carrier spacing. Reduced Inter-SymbolInterference (ISI) is shown by increased symbol spacing per frequency.This ensures that consecutive OFDM symbols subcarriers do not use thesame frequency and the adverse effects from multipath delay spread areavoided. The same Instantaneous Signal-to-Noise ratio (SNR) as a singlecarrier is also illustrated.

There now follows a description of a flexible OFDM communications systemthat can incorporate the low PAPR preamble in accordance with anon-limiting example of the present invention. The flexible OFDM systemis typically full-duplex and has various types of supported modems,including an IEEE 802.11A modem with a maximum data rate of 54 Mbps andIEEE 802.16E modem with a maximum data rate of about 74 Mbps. Somecustom modems such as produced by Harris Corporation have a data rate ofabout 400+Mbps and can be supported. The spectral efficiency can equalthe data rate/bandwidth such as for a 16-QAM that is equal to about143.3 Mbps/52 MHz as 2.76 bits/sec/Hz or 64-QAM as 215.0 Mbps/52 MHz as4.13 bits/sec/Hz. The flexible OFDM system can use a programmableinterleaver and programmable guard interval with an increase of thecyclic prefix for more multipath tolerance and reduced cyclic prefix forless overhead and increased data rate when channel conditions permit.

The flexible OFDM communication system can use a programmable FFT/IFFTin supported FFT sizes of 64, 256, 1024, 2048 and 4096. It hasprogrammable subcarriers with an FFT mapper and a variable number ofdata carriers and variable number of pilot tones and variable subcarrierlocations with the programmable preamble and channel estimation. Itallows a PAPR reduction with the LPI/LPD/AJ and fast frequency hoppingvia the FFT mapper. It includes frequency-domain spreading using aspreading transform with a power amplifier linearization.

The flexible OFDM can use selectable hardware coding and trade FPGAslices versus coding gain in a turbo decoder, Viterbi decoder and withprogrammable code rates. A programmable window function can have atrade-off window size for multipath robustness versus spectral bandwidthand selectable hardware bit-widths that trades FPGA slices versusperformance. For this reason, the low PAPR preamble is advantageous. Anexample of simulation code is shown below:

% Title: GenLowPAPRCarriers_Tester.m % Author: Chris Moffatt % Program:OFDM Study % Date Created: 5/6/2005 % Description: Generate Low PAPRSinusoids % Input Parameters: BANDWIDTH=Desired Bandwidth, Fs=SamplingFrequency, N=Number of Samples%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% close all; clear; clc   %Close all Matlab figures, clear Matlab program space memory,   cleartext in Matlab command window fprintf(‘\n’)%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%% PROGRAMCONTROL PARAMETERS %%%%%%%%%%%% PLOTTING = ‘ON’; % [ON / OFF] Fs =100e6; % 100 MHz Sample Rate % NOTE: USE AN EVEN NUMBER OF CARRIERS Nc =52; % Number of Carriers used to Generate   Impulse%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%   %%%%%%% %%%%GENERATE TIME-DOMAIN, N-CARRIER, QUADRATIC-PHASE COMPLEX ENVELOPE%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%% Apply PAPR Reductionf = 1:Nc; % Frequency of each subcarrier fc = (Nc + 1) / 2; % CenterFrequency cw = 2*pi*(f − fc); % Radian Frequency Offset (with   respectto center frequency) QuadraticPhases = (pi/Nc) * (f−fc).{circumflex over( )}2; % Vector of Quadratic Phases (one for   each carrier) %%%% MapQuadratic Phases to their Frequency Carriers via IFFT FFT_SIZE =2{circumflex over ( )}cell(log2(Nc)); ComplexPhases =exp(j*QuadraticPhases); ifft_in = [0 ComplexPhases(Nc/2+1:end)zeros(1,FFT_SIZE-Nc-1) ComplexPhases(1:Nc/2)]; z = ifft(ifft_in,FFT_SIZE); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%% Plot Results if(strcmp(PLOTTING,‘ON’) == 1) % Check if plotting   function turned on %%%% Calculate Measured PAPR and Theoretical PAPR  z_mag = abs(z); Pavg = mean(z_mag.{circumflex over ( )}2);  papr = (z_mag.{circumflexover ( )}2) / Pavg;  PAPR = 10*log10(max(papr));  theoretical_papr =10*log10(Nc);  crlf; fprintf(‘PAPR of %d Sinusoids Added In-Phase =%f’,Nc,PAPR)  crlf; fprintf(‘Theoretical PAPR of %d Sinusoids AddedIn-Phase = %f’,Nc,theoretical_papr);   crlf; crlf  figure,plot(QuadraticPhases), title(‘Quadratic Phases’)  figure, plot(z_mag),title([‘Complex Envelope (Amplitude) of ’ num2str(Nc) ‘ Carriers’]),  xlabel(‘Sample’)  figure, plot(10*log10(papr)), title([‘PAPR ofComplex Envelope in dB of ’ num2str(Nc) ‘   Carriers’]),ylabel(‘PAPR(dB)’)  figure, plot(fftshift(abs(fft(z)))), title(‘ImpulseResponse − Frequency Domain’)  figure, pwelch(z, [ ],[ ],[ ],Fs); Plot_CCDF_Curve(z), title(‘CCDF of Low PAPR Signal’) end%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Referring now to FIGS. 6 and 7, there are illustrated respectivefunctional block diagrams for a transmitter 100 (FIG. 6) and a receiver200 (FIG. 7) that provides flexible OFDM and incorporates use of the lowPAPR preamble in accordance with non-limiting examples of the presentinvention. The receiver also includes a channel estimator circuit asexplained below and used with the low PAPR preamble for channelestimation such as channel sounding. The transmitter 100 in thisnon-limiting example as illustrated applies a frequency hoppingalgorithm to OFDM subcarriers and frequency domain spreading, forexample a Walsh transform, before an IFFT circuit and has circuitry andfunctional algorithms for generating the low PAPR preamble.

Many of the high-level components of the illustrated transmitter 100 andreceiver 200 are functionally similar to the components shown in theprior art modem of FIGS. 1A and 1B, but with further details andfunctional block components added to the transmitter and receiver blockdiagrams shown in FIGS. 6 and 7 and with capability to incorporateflexible OFDM and the low PAPR preamble. For reference purposes, thedescription for the transmitter begins with reference numerals in the100 series and the description for the receiver begins with referencenumerals in the 200 series.

Functional components that aid in generating the frequency hopping, OFDMsignal that can be Walsh transformed in accordance with a non-limitingexample include a Pseudo-Random Amplitude and Phase Generator 102 andPseudo-Random Subcarrier Locations circuit 104 as generating inputs to amodulation and mapping circuit. Both the Pseudo-Random Amplitude andPhase Generator 102 and Pseudo-Random Subcarrier Locations circuit 104are operative with an Encryption Algorithm 106 and a Cryptographic andKey generator circuit (Crypto-Key) 108 and Master Clock 110. Thesecomponents can be generally referred to collectively as an encryptedpseudo-random signal generator A Frequency Domain Spreader circuit 112is located before an IFFT circuit, as illustrated and explained below,and is operable for frequency spreading the signal, such as by applyinga Walsh transform. Also, a digital/analog converter can receive a signalfrom a Bandwidth Adjust DAC Sample Rate circuit 114 for removingspectral lines. These components are explained in further detail below.

As illustrated in FIG. 6, a signal is received within a data buffer 120and passes through a CRC generator 121 and data scrambler 122. An FECencoder circuit shown by the dashed lines at 124 can include a ForwardError Correction encoder 126, for example, a convolutional encoder andpuncturer circuit 128. The encoded signal is interleaved within aninterleaver circuit 130. The signal passes into a modulation and symbolmapping circuit shown generally by the dashed lines at 132. Thismodulation and symbol mapping circuit 132 includes a QAM/PSK modulator134 and Insert Pilot Carriers and PAPR Reduction Carriers circuit 136that inserts pilot carriers and PAPR reduction carriers into the signal.A circuit can generate a preamble sequence and insert the sequence to bereceived with selected preamble subcarriers and quadratically modulated.The circuit 136 could include a preamble sequence generator forgenerating a preamble sequence as a training sequence explained ingreater detail below. Carriers are mapped to the IFFT in a matrixoperation in a subcarrier mapper circuit 138.

The Encryption Algorithm 106 is operative not only with the Crypto-Keycircuit 108 and the Master Clock 110, but also the Pseudo-RandomAmplitude and Phase Generator 102, which generates pseudo-random signalsto the QAM/PSK Modulator 134 in accordance with a non-limiting exampleof the present invention. The Pseudo-Random Subcarrier Location circuit104 is also operative with the Subcarrier Mapper circuit 138 andreceives signals from the Encryption Algorithm 106. The OFDM Subcarriersare frequency hopped quickly by means of such circuits.

In accordance with a non-limiting example, the Frequency Domain Spreadercircuit 112 is located before the IFFT circuit 140 and applies the Walshtransform or other linear operator transform in the frequency domain. Ifthe Frequency Domain Spreader circuit 112 were located after the IFFTcircuit 140, then the Walsh or other function would force a time-domainspreading. It should be understood that the Frequency Domain Spreadercircuit 112 and IFFT circuit 140 can typically be considered with themodulation and mapping circuit 132 as an OFDM modulation circuit or OFDMmodulation and mapping circuit. In accordance with a non-limitingexample, the spreading resulting from application of the Walsh transformoccurs in the frequency domain. A cyclic extension as a guard intervalis then added within a Cyclic Extension circuit 142. A symbol-shapingfilter 144 such as a Finite Impulse Response (FIR) filter, cosinefilter, or raised cosine filter can be operative as a “Time Window” forsymbol shaping in conjunction with the Cyclic Extensions. A packetbuffer 146 receives the signals, and after buffering, the signals areconverted to analog signals in a digital/analog converter 148. The D/Aconverter 148 also receives from a Bandwidth Adjust DAC Sample Ratecircuit 114 a signal for further processing that removes spectral lines.The D/A Converter 148 passes signals to a Radio Integrated Circuit (IC)Wideband Slow Frequency Hopping circuit 150. The RF carrier can besubjected to a pseudo-random frequency hopping algorithm for enhancedbandwidth, and is operative also as a frequency up-converter, asillustrated.

Basic components of the frequency up-converter circuit 150 can include atransmit chain circuit 152 that receives the signal into a mixer 154.The signal passes through a Bandpass Filter 156, a series of amplifiers158 and through a Single Pole Double Throw (SPDT) switch 160. Afterswitching, a low pass filter 162 filters the signal. The radio frequencysignal is amplified by the Power Amplifier 164 for subsequenttransmission through antenna 166. Other components in the circuit 150include a phase-locked loop circuit 170, a 40 MHz signal generator 172as a non-limiting example, a low pass filter 174, an amplifier 176, asynthesizer 170, another amplifier 180, a bandpass filter 182, a summercircuit 184, and another amplifier 186 that connects to the mixer 154.The component parts of the frequency upconverter circuit 150 may be usedto affect a low rate frequency hopping scheme, where the entire OFDMbaseband waveform is frequency translated to different centerfrequencies. Such slow frequency hopping can further guard againstinterference and provide an additional level of encryption if the slowhopping sequence is designed as such.

The transmitter 100 as described is a non-limiting example and othertypes of transmitters could be used. It should be understood that withadvances in DSP and other circuit functions, processing can possiblyoccur at baseband. The low PAPR preamble is processed and part of thissignal. As will be explained, the low PAPR preamble, in accordance withnon-limiting examples, uses quadratic phases applied to each activesubcarrier in order to achieve a typical PAPR of about 2.6 dB.

It should also be understood that the subcarrier mapper circuit 138 mapscarriers to the IFFT circuit 140. For example, if the IFFT circuit 140has an input with a 64 sample signal in the frequency domain, it wouldgive a 64 sample signal in the time domain as a matrix operation. Thesubcarrier mapper circuit 138 can change the order of the vectors toposition symbols on arbitrary subcarriers and apply zero to othersubcarriers. For example, some of the samples in a 64 sample vectorwould be zeros, meaning they would not show up in the frequency domainif they are OFF. Any that are ON or non-zero will change location withevery IFFT cycle (once per symbol) to produce the frequency hopping OFDMsignal. The nature of the frequency hopping for the OFDM signal isgenerated by the Encryption Algorithm 106 and the Pseudo-RandomSubcarrier Locations circuit 104 and the Pseudo-Random Amplitude andPhase Generator 102. The QAM/PSK Modulator 134 aids in producing theconstellation amplitude and phase.

One of the aspects could involve obscuring to an unintended receiverthat the data has been encrypted. To obscure the encryption, threeunknowns are produced by the transmitter. For example, there is a) theunknown of the transmitted amplitude and phase; b) the unknown of thepseudo-random amplitude and phase; and c) the unknown of the channelamplitude and phase. Because there are three unknowns, it is notpossible to know which signal is transmitted with an encryptionalgorithm, based on the Cryptographic Key and Master Clock,

The frequency domain spreader circuit 112 operates as a matrixoperation. For example, if a 64 IFFT circuit 140 is employed, then a64×64 Walsh matrix (as a non-limiting example) can be used tofrequency-spread the subcarriers and provide processing gain. An inputvector would be multiplied by the Walsh matrix. It should be understoodthat a Walsh matrix is a square matrix with dimensions that can be apower of “two.” The entries are positive or negative one (+1, −1). TheWalsh matrix can be obtained from a Hadamard Matrix that is defined by arecursive formula of the same dimension by arranging rows such that thenumber of sign changes is in increasing order, i.e., sequentialordering. Each row of a Walsh matrix corresponds to a Walsh function.The ordering of rows in a Walsh matrix can be derived from ordering aHadamard matrix by applying a bit-reversal permutation and a Gray codepermutation. The Walsh functions form an orthogonal basis of a squarethat is integratable on a unit interval. Thus, it can generatestatistically unique sets of numbers suitable for use in encryption,also known as “pseudo-random and noise codes.” The multiplication may beimplemented efficiently as a series of additions and subtractions.

The Bandwidth Adjust DAC Sample Rate circuit 114 is operative with theD/A converter 148 and can adjust the sample rate and remove spectrallines. As a result, it is harder to detect the waveform with aSpectrogram. It should be understood that the transmitter 100 asdescribed is operative to form a Frequency Hopping OFDM signal with aWalsh transform. For example, if an IFFT is used with 64 samples persymbol, the frequency location of each subcarrier can be changed every64 samples. As an example, if an IFFT is computed every fourmicroseconds, then frequency hopping on all 64 carriers can occur everyfour microseconds to impart a fast hopping rate. Because this can beaccomplished symbol-by-symbol, the frequency hopping OFDM communicationssystem as described can also be termed a Symbol-Based Randomized OFDMbecause the subcarrier frequency locations are randomly changed. Anotherreceiver would not be able to determine the subcarrier locations withoutthe Encryption Algorithm and related circuits, and a fullsynchronization.

FIG. 7 shows a high-level functional block diagram of a receiver 200that can be used for processing the OFDM communications signal thatincorporates the low PAPR preamble in accordance with a non-limitingexample of the present invention and includes an added channel estimatorcircuit 237 as part of the demapping and demodulation circuit. Similarcomponents that are used in the block diagram of FIG. 6, such as anEncryption Algorithm circuit, Cryptographic Key circuit, Master Clock,Pseudo-Random Amplitude and Phase Generator, Pseudo-Random SubcarrierLocations circuit, and Bandwidth Adjust ADC Sample Rate circuit aregiven similar reference numerals as used in FIG. 6, except they are nowplaced in the 200 series. This receiver circuit 200 includes aSymbol-Based Subcarrier Synchronization circuit 216. It also uses aFrequency Domain Despreader circuit 212 instead of a Frequency DomainSpreader circuit 112 as in the transmitter 100 of FIG. 6.

Other high-level components illustrated for this receiver circuit 200include an antenna 220, a low noise amplifier (LNA) 222, and RadioIntegrated Circuit down-converter 224, which can process a frequencyhopping carrier signal in reverse if it had been processed for widebandby the Radio IC Wideband Slow Frequency Hopping circuit 150 shown in thetransmitter 100 of FIG. 6. The Analog/Digital Converter 226 receives anIF or baseband signal from the down-converter 224, and a signal from theBandwidth Adjust ADC Sample Rate circuit 214 and reverses the processused at the transmitter 100. The signal is forwarded to the Data Buffer228 and Symbol-Based Subcarrier Synchronization circuit 216, whichsynchronizes the subcarriers for further processing using, for example,the low PAPR preamble. The Guard Interval circuit 230 removes the guardinterval and the signal is processed with the Fast Fourier Transform asan OFDM demodulator in an FFT circuit 232. The Inverse Walsh Transformis applied in an Inverse Walsh Transform circuit 212. A subcarrierdemapper and demodulation circuit is shown by dashed lines at 234 andperforms an inverse mapping operation to the subcarriers in subcarrierdemapper circuit 236.

This circuit is followed by the channel estimation using the channelestimator 237, operative with the incorporated low PAPR preamble such asfor channel sounding and can operate with a channel estimate feedbackcircuit that can transmit data regarding the communications channel backto the transmitter. This is advantageous to determine the usefulduration of channel adaptation information and overcome the problemswhen propagation channels change rapidly due to Doppler or other factorsrelated to the coherence time of the channel such as the time domain andDoppler spread, where the Doppler spread and coherence time areinversely proportional to one another and characterize the time varyingnature of the frequency disbursiveness of a channel in the time domain.In addition, the transmitter can use the feedback information toadaptively select subcarriers for transmission and change subcarriermodulation for adaptive interference mitigation. Various amounts ofspreading can be applied to the subcarriers depending on the amount ofprocessing gain required to overcome the intentional or unintentionalinterference. Additional spreading can be applied to mitigate frequencyselective fading based on the channel conditions indicated by thefeedback signal. Typically, the symbol duration is much longer than thelength of a channel impulse response so that inter-symbol interferenceis avoided by inserting a cyclic prefix for each OFDM signal. Additionalperformance can be extracted in the presence of channel stateinformation at the transmitter such that knowledge of a channel at thetransmitter can be used to improve link performance, reliability andrange. Thus, channel knowledge at the transmitter provides the abilityto determine the quality of a channel response across the signalbandwidth to select the best portion of the band on which to transmit.The channel knowledge can be used by a base station or other transmitterto transmit data streams to multiple subscribers on the sametime-frequency resources. Sometimes a subscriber or other receiver canmeasure the downlink (DL) channel using the channel estimator 237 andtransmit a feedback message that could contain information enabling thebase station to perform some type of closed-loop transmission on thedown link. Given a reciprocal link, it is also possible to transmitchannel sounding waveforms on the up link to enable the base station todetermine the base station to subscriber station channel response.

The pilot tones are removed in a pilot remove circuit 238 and the signalis demodulated in a Symbol to Number (QAM/PSK) Demodulator circuit 240.The deinterleaver circuit 242 deinterleaves the signal. A decodingcircuit is shown by dashed lines at 244 and is operative fordepuncturing within depuncture circuit 246 and Forward Error Correction(FEC) decoding such as Viterbi decoding within a FEC decoder such as aViterbi decoder 248. Data descrambling occurs at a Data Descrambler 250,followed by data buffering in data buffer 252 and processing for a CRCcheck by CRC circuit 254.

The transmitter 100 and receiver 200 shown in FIGS. 6 and 7 can generateand receive a signal that is a fast-carrier frequency hopping signal.This hopping can be much faster than a conventional Bluetooth systemthat hops frequencies at 1600 hops/second over a 80 MHz radio frequencybandwidth using a single carrier having a 1 MHz bandwidth. It shouldalso be understood, for example, as shown in the graphs of FIG. 4, thata change in signal-to-noise ratio (S/N) can be based on the number ofsubcarriers and can be used as a method of varying the range of theinstantaneous subcarrier signal-to-noise ratio versus that data rate inan adaptive wireless communications system.

For example, the receiver 200 could measure the received Signal-to-Noiseratio per subcarrier, for example, by using channel estimation symbols,the low PAPR preamble, or a special channel estimation packet. Inaccordance with a non-limiting aspect, the low PAPR preamble isadvantageous for this purpose. Information can be passed back to thetransmitter as a “channel mask,” specifying the number of subcarriers to“turn-off” and the possible frequency locations of interferers aschannel impairments such that the transmitter 100 could use thenegotiated channel mask to avoid transmission on any undesirablefrequencies. In one example, ten carriers are turned ON simultaneouslyover a 100 MHz bandwidth, and each carrier is transmitted for 640nanoseconds (corresponding to a 1/FFT rate), such that each carrier canhop 1,562,500 times per second. This is about a one thousand timesfaster hopping than the Bluetooth protocol and can provide more than tentimes the data rate.

The transmitter 100 can create multiple subcarriers located at specificfrequencies and can generate a pseudo-random frequency hop for eachsubcarrier frequency by applying the frequency hopping algorithm asexplained before. The IFFT circuit 140 creates multiple subcarrierslocated at specific frequencies. In accordance with a non-limitingexample, only a small subset of all possible subcarriers need to be usedat any one time, although all subcarriers can be used if necessary. Forexample, as in the example discussed above, instead of 64 subcarriers,only 10 subcarriers can be used in this non-limiting example, giving inthat example the 1,562,500 hops per second.

The subcarrier center frequencies can be changed at the OFDM symbol rateusing the encryption algorithm for the pseudo-random frequencies. Thisoccurs at the modulation and mapping circuit 132 in which the carriersare mapped to the IFFT. The center frequencies of the subcarriers canappear random because of the frequency hopping algorithm. The symboltime duration can be very short as noted above, and therefore, eachsubcarrier would appear for a short time at any specific frequency.

In some aspects, the guard time can be reduced or eliminated by ensuringthat consecutive symbols do not contain subcarriers at the samefrequency location. For example, in prior art systems, if two symbolsare back-to-back on the same frequency, multipath signals could arriveat different times at the same location. By using the system andcircuits shown in FIGS. 6 and 7, these signals do not appear on the samefrequency and the signal would typically not be affected by multipath,thus preventing Inter-Symbol Interference (ISI) and substantiallyreducing required guard time, reducing transmission overhead, andincreasing data rate.

It is possible using the transmitter 100 and receiver 200 as shown inFIGS. 6 and 7 to eliminate or substantially reduce guard time, e.g.,“guard interval.” Also, it should be understood that an additional guardcan be added by modifying the frequency hopping algorithm such that nofrequency can be used twice in a row for consecutive symbols, andthereby preventing Inter-Symbol Interference (ISI) because of multipathchannel effects. As noted before, this eliminates or substantiallyreduces the required guard interval, reduces transmission overhead, andincreases the data rate.

It is also possible to dynamically add and remove subcarriers dependingon the required data rate for an “on-the-fly” adaptation as a flexibleOFDM system. The minimum carrier spacing can increase to reduce theInter-Carrier Interference (ICI) and provide robustness to jamming i.e.anti-jamming (AJ) capability, because of the frequency hopping signal.As long as carriers are not transmitted next to each other in thefrequency domain, the Inter-Carrier Interference will be reduced.

It is also possible for the carrier frequency to hop pseudo-randomly andcover a wide bandwidth. This can be accomplished by the Radio ICWideband Slow Frequency Hopping circuit 150 shown in FIG. 6 andoperative as a frequency up-converter circuit.

A “dead-time” pseudo-random generator can be introduced into the systemto decrease “ON” time, and the output spacing between symbols can beincreased. The spacing can be varied using the pseudo-random generatorto prevent spectral lines and reduce cyclostationary statistics of thesignal. This type of system can be implemented without an output samplecontrol. The system can wait a random amount of time beforetransmitting. By removing the spectral lines, it is more difficult forother systems to detect the transmitted communications. The termcyclostationary can refer to the mean standard deviation as a secondorder statistic of the signal. The output sample control can refer tocontrol at the D/A Converter 148.

The subcarrier constellation amplitude and phase values can also bevaried pseudo-randomly using the generator 102 operative with themodulator 134 as shown in FIG. 6. For example, pseudo-random amplitudeand phase values can be generated using the encryption algorithm. Thepseudo-random amplitude and phase values can be added to the intendedamplitude and phase values before transmission. By adding thepseudo-random amplitude and phase values to each subcarrier, the symbolconstellation is no longer a standard QAM/PSK. If the transmitter signalis detected by an unintended receiver, that receiver would not be ableto demodulate the signal because there would be too many unknowns. Forexample, the transmitted or intended amplitude and phase would be anunknown, together with the pseudo-random amplitude and phase that isadded to the signal, and a further unknown being the channel amplitudeand phase for the multipath. This results in three unknowns. Thepseudo-random amplitude and phase values would appear as a typicalrandom channel effect to the unauthorized or unintended receiver.

It should be understood that these algorithms can be added to SoftwareDefined Radios (SDR) and can be implemented with some changes to varydata rate and modulation. The data rates, bandwidth, transmission powerand LPI/LPD performance can be improved by varying the subcarriermodulation scheme, sample rate, IFFT size, IFFT duration and the numberof subcarriers used per OFDM symbol.

As shown in FIG. 6, a Walsh transform can be applied in the frequencydomain for frequency spreading, since it is applied before the IFFTcircuit 140 using the Frequency Domain Spreader circuit 112. It is knownthat Walsh transforms are typically used in communications systems suchas CDMA for time-domain spreading and for creating orthogonal codes formultiple access schemes. The Walsh Transform can be used in the system,apparatus, and method as illustrated to spread subcarriers over thefrequency domain. This can provide a significant reduction in theaverage power (dBm/Hz/sec) for enhanced LPI/LPD performance, allowingmore transmit power within the same FCC spectral mask and reducing theeffect of Frequency Selective Fading by providing a frequency-domainprocessing gain. It also provides additional anti-jamming (AJ)robustness. Also, out-of-band noise (OBN) emissions can be reducedsimilar to time-windowing because of the steeper “roll-off” caused bythe Walsh transform. The Walsh transform as a matrix is made up of onlypositive and negative ones (+1, −1) and requires only additions andsubtractions, and no multiplications. This would allow a trade-off forthe number of carriers versus the data rate versus the transmit powerand distance for the same FCC spectral mask. In the Walsh transform,matrix rows can be exchanged with each other. The transform would stillbe orthogonal at the receiver 200. These row permutations can beperformed to increase further the LPI.

It should be understood that OFDM is susceptible to Frequency SelectiveFading because of multipath. The Walsh transform can provide processinggain to the system and robustness against frequency selective fading.

The system, apparatus and method as described provides a very fastfrequency hopping by changing subcarrier frequency locations, forexample, at the OFDM symbol rate. Thus, it can provide a reducedspectral density over time (decibel/hertz/second) in order to provide aLow Probability of Interception (LPI) and Low Probability of Detection(LPD). The system as described is much faster than Bluetooth systems,and makes the transmission within the FCC spectral mask possible. Italso eliminates or substantially reduces a guard interval by ensuringthat subcarriers do not appear on the same frequency for consecutiveOFDM symbols. The system also provides robustness against Inter-SymbolInterference (ISI) due to multipath. The Walsh transform can be appliedin the frequency-domain to spread the frequency-hopping subcarriers overthe spectrum and reduce the power spectral density (decibels over hertz)to improve LPI/LDP performance or help comply with FCC spectral maskrequirements. It can also provide a processing gain against frequencyselective fading and provide robustness against jamming.

In accordance with a non-limiting example, the low PAPR preamble is usedfor channel estimation in one non-limiting example. The low PAPRpreamble has a PAPR that typically does not exceed 2.6 dB in onenon-limiting example, regardless of the number of subcarriers.Typically, the subcarriers are evenly spaced and have equal amplitude,which is ideal for channel sounding. The result is a low PAPR preamblethat operates over a variable number of subcarriers and improves SNR.Both simulations and measurements demonstrate significant benefits ofthe proposed low PAPR preamble in long-range wireless communicationsapplications that use an OFDM waveform for radio communication.

The low PAPR preamble, in accordance with a non-limiting aspect, has atypical PAPR of 2.6 dB in a non-limiting example, regardless of thenumber of subcarriers. This low PAPR preamble requires less back-offfrom the P1 dB point of a power amplifier typically used in thecommunications circuit, allowing higher transmit power. This enhancedtransmit power during the preamble results in improved channelestimation at the receiver. In addition, more SNR for channelequalization allows a lower BER. In communications systems where ahigher SNR is required for detection than for data symbol decoding (dueto robust Turbo/LDPC FEC coding schemes), this technique using the lowPAPR preamble provides higher SNR for detection and provides lower PAPRthan existing preamble techniques. In addition, the low PAPR preambleprovides good correlation properties for detection and an efficientimplementation using IFFT/FFT. The subcarriers are typically evenlyspaced and have equal amplitude, ideal for channel sounding. The lowerPAPR preamble results in the use of power amplifiers having a lowercost, Size, Weight, and Power (SWaP).

Both the commercial and the government (military) market are demandingcommunications systems with higher data rates and an increased spectralefficiency (i.e., increased bandwidth utilization). Increasing the datarates and spectral efficiency for OFDM involves increasing the number ofsubcarriers. The larger number of subcarriers leads to higher PAPR,which leads to larger power amplifiers, consuming more cost, Size,Weight, and Power (SWaP). Being independent of the number ofsubcarriers, this low PAPR preamble allows reduced preamble PAPR,provides optimal channel estimation, good correlation properties fordetection, and efficient implementation via IFFT/FFT.

It should be understood that OFDM preambles are used for many purposes.FIG. 8 is an example of an OFDM preamble such as established by IEEE andshown generally at 260. The shorter sync symbols 262 are used for signaldetection, Automatic Gain Control (AGC), antenna diversity selection,coarse frequency offset estimation, and timing synchronization. Thelonger sync symbols 264 provide channel estimation and fine frequencyoffset estimation. For channel estimation, it is desirable to have asample for each subcarrier at the center of its frequency bin usingevenly spaced, equal amplitude subcarriers. The resulting fluctuationsin the preamble's received nominally flat-topped spectrum reflect thefrequency response of the channel.

There now follows a general description of techniques to apply thequadratic phase to obtain the low PAPR preamble in accordance withnon-limiting examples. General calculations and methodology are firstexplained and followed by a more detailed description relative togenerating and processing the low PAPR preamble. It is well known thatthe addition of N sinusoids produces a time-domain waveform with acertain PAPR. By setting the phase of each sinusoid to a specific anglerit is possible to produce a symbol or waveform with minimal PAPR. Byapplying quadratic phases to each subcarrier, a typical PAPR of 2.6 dBcan be achieved. Depending on the number of subcarriers, the value ofPAPR using the quadratic phasing as described below can slightly vary.The typical range of PAPR values depends on the number of subcarriersand usually ranges from 2.9 dB for low number of subcarriers to 2.6 dBfor a large number of subcarriers. As the number of subcarriersincrease, the value stabilizes about 2.6 dB PAPR. This technique isconsidered distortionless since only phase values of the sinusoids areadjusted and no clipping, filtering, or companding of the time-domainwaveform is applied. It is still possible to achieve lower PAPR throughsignal distortion techniques at the cost of in-band and out-of-bandinterference.

Consider N_(c) orthogonal complex exponentials with identical amplitudeand equally spaced Δf=1/T_(sym) Hertz apart, where T_(sym) is the symbolduration. The output multicarrier waveform is formed by the summation ofthe complex exponentials (Equation 1):

${x(t)} = {\sum\limits_{k = 0}^{N_{c} - 1}{A_{k} \cdot ^{- {j{({{2{\pi {({f_{k} - f_{c}})}}t} + \varphi_{k}})}}}}}$

Where f_(k) are the subcarrier center frequenciesf_(k)=f_(o):Δf:f_(max), f_(o) is the fundamental (lowest) subcarrierfrequency, and f_(max) is the highest subcarrier frequency. Thefrequency f_(c) is used to center the complex spectrum about the origin(Equation 2).

$f_{c} = {{\frac{f_{\max} + {f_{o}1}}{2}\mspace{14mu} {and}\mspace{14mu} f_{\max}} = {{\Delta \; {f \cdot \left( {N_{c} - 1} \right)}} + f_{o}}}$

In general, F_(s)≧2·f_(max), T_(s)=1/F_(s), T_(sym)=1/Δf,N=N_(sym)·(F_(s)·T_(sym)) and t=0:T_(s):T_(s)·N−T_(s), where N_(sym) isthe number of symbols to generate. The maximum PAPR that occurs when allN_(c) subcarriers are added in phase is (Equation 3):

PAPR_(max)=10 log(N _(c))(dB)

The ideal phase relationship for producing the minimum PAPR is obtainedby passing the zero-phase complex envelope of the summed complexexponentials through the transfer function (Equation 4):

${H(\omega)} = ^{- {j(\frac{\omega^{2}}{4\alpha})}}$ where$\alpha = \frac{N_{c}\pi}{T^{2}}$

This phase relationship provides (Equation 6):

PAPR_(typ)=2.6 dB

independent of the number of carriers and frequency spacing. ¹Note thatwhen using an odd number of symbols N_(sym) and analyzing the spectrumwith an FFT, add an extra ±½ to f_(c) to allow subcarriers to fall ontointeger frequency bins in order to avoid Inter-Carrier Interference(ICI).

The new output multi-carrier waveform with PAPR of about 2.6 dB isformed by applying the quadratic phases (Equation 7):

$\begin{matrix}{\varphi_{k} = \frac{\omega^{2}}{4\alpha}} \\{{= {\frac{\pi}{N_{c}}\left\lbrack \frac{\left( {f_{k} - f_{c}} \right)}{\Delta \; f} \right\rbrack}^{2}},}\end{matrix}$ where  Δ f = 1/T

The continuous-time low PAPR preamble signal is found by substitutingequation (7) into equation (1) to yield (Equation 8):

${x(t)} = {\sum\limits_{k = 0}^{N_{c} - 1}{A_{k} \cdot ^{j \cdot {({{2{\pi {({f_{k} - f_{c}})}}t} + {\frac{\pi}{N_{c}}{\lbrack\frac{({f_{k} - f_{c}})}{\Delta \; f}\rbrack}}^{2}})}}}}$

The discrete preamble signal x(n) can be generated using an IDFT bycentering the subcarriers about DC on integer subcarrier centerfrequencies and setting Δf=1 (Equation 9):

$\begin{matrix}{{x(n)} = {\frac{1}{N} \cdot {\sum\limits_{k = {- \frac{N_{c}}{2}}}^{\frac{N_{c}}{2} - 1}{A_{k} \cdot ^{j \cdot {({\frac{2 \cdot \pi \cdot k \cdot n}{N} + {\frac{\pi}{N_{c}}k^{2}}})}}}}}} \\{= {\frac{1}{N} \cdot {\sum\limits_{k = {- \frac{N_{c}}{2}}}^{\frac{N_{c}}{2} - 1}{\left( {A_{k} \cdot ^{j{({\frac{\pi}{N_{c}}k^{2}})}}} \right) \cdot ^{j \cdot {(\frac{2 \cdot \pi \cdot k \cdot n}{N})}}}}}}\end{matrix}$

where N is the IDFT size and n=0:N−1. The low PAPR preamble is nowcomputed efficiently using an IFFT (Equation 10):

${x(n)} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{X_{k} \cdot ^{j \cdot {(\frac{2 \cdot \pi \cdot k \cdot n}{N})}}}}}$

where the complex quadratic subcarrier values are (Equation 11):

X _(k) =A _(m) ·e ^(j·φ) ^(m)

with quadratic phase (Equation 12):

$\varphi_{m} = \left\{ \begin{matrix}{\frac{\pi}{N_{c}} \cdot m^{2}} & {{- \frac{N_{c}}{2}} \leq m < \frac{N_{c}}{2}} \\0 & {elsewhere}\end{matrix} \right.$

and amplitude (Equation 14):

$A_{m} = \left\{ \begin{matrix}A & {{- \frac{N_{c}}{2}} \leq m < \frac{N_{2}}{2}} \\0 & {elsewhere}\end{matrix} \right.$

The discrete frequency m preorders the IFFT input data (i.e. performs anFFT shift) and is obtained using the modulo function (Equation 14):

$m = {{{mod}\left( {{k + \frac{N}{2}},N} \right)} - \frac{N}{2}}$

Furthermore, by application of this quadratic phase relationship, thelocation where the maximum peak occurs in time becomes deterministic.The maximum PAPR of y(t) occurs at approximately (Equation 6):

$t = \left( {{\pm \left( \frac{T}{2} \right)} - {1.163 \cdot \frac{T}{\sqrt{2N}}}} \right)$

Simulations have been run in order to verify creating multi-carriersignals that possess a PAPR of about 2.6 dB, independent of the numberof carriers. The sinusoids produce maximum PAPR when added together withidentical phases and produce a PAPR of about 2.6 dB when added togetherwith optimum quadratic phases. These two cases are compared throughsimulation using 64-carriers (complex sinusoids).

FIG. 9 shows 64-complex sinusoids in the frequency domain. Thesubcarriers are evenly spaced in frequency for the quadratic phasecalculations to produce the desired result.

The 64-complex sinusoids for 64 carriers (zero-phase) were first summedtogether with identical phases, as shown in FIG. 10. FIG. 11 shows thecomplex envelope in log magnitude of the PAPR for 64 carriers(zero-phase) in dB. For the zero-phase case, a PAPR 18.061800 dB wasmeasured in the simulation from the 64-sinusoids added in-phase. Thiscompares identically to the theoretical PAPR of 64 sinusoids addedin-phase 18.061800 dB. FIG. 12 shows the quadratic phases calculatedusing Equation 5 listed above.

The PAPR of 64-sinusoids summed together using quadratic phases wasmeasured from simulation to be 2.600510 dB. This is very close to thepredicted theoretical minimal PAPR of about 2.6 dB. The maximum PAPRlocation measured from the simulation occurred at 0.000391 seconds,which is close to the theoretical location of 0.000397 seconds predictedby Equation 6 listed above.

FIG. 13 compares the PAPR of the complex sinusoids added together withidentical phases for maximum PAPR and optimum quadratic phases forminimal PAPR. The maximum PAPR is approximately 18 dB on theidentical-phase multi-carrier signal and approximately 2.6 dB on thequadratic phase multi-carrier signal as illustrated on the graphs.

Although the simulation was run with identical amplitudes for eachcarrier, the PAPR can still be reduced for other modulations schemes,such as AM, where the amplitude is not constant. FIG. 14 is a graph thatillustrates using a random uniform amplitude distribution applied toeach of the 64-carriers. Each carrier was independently AM modulatedwith a random amplitude value between 0 and 1. This simulation ran for300 iterations, during which the PAPR statistics were collected for boththe identically phased (zero-phase) carriers and the quadratic-phasecarriers. This shows that on average, the PAPR was improved for thequadratic-phase case by 11.15 dB. It should be noted however, there waslittle to no improvement when random phase modulations were applied.This resulted because the quadratic phase relationship no longer holdsfor the phase-modulation case.

There now follows more details regarding the low PAPR preamblegeneration and processing and basic description for purposes ofunderstanding. Digital communication systems using burst waveformsrequire channel estimation for each separate transmission. For such apacket waveform with short transmit durations, the channel impulseresponse is assumed to be stationary throughout. Typical data burststherefore precede the payload data with training sequences to facilitatethe channel estimation operation. An example training sequence (orpreamble) is shown in FIG. 8. The short sync segments t₁-t₁₀ (262)differ from the long sync symbols T₁ and T₂ (264) (including the doubleguard interval GI2).

The receiver, such as shown in FIG. 7 and also described relative toFIG. 18, uses the short sync symbols in the preamble to detect thearrival of the packet, recover the timing and perform coarse and finefrequency offset estimation (fine estimation is done using the long syncsymbols). The received OFDM waveform is then converted to the frequencydomain (using an FFT). Channel estimation and compensation is thenperformed using the long sync symbols 264 in the preamble. The pilottones embedded in OFDM symbols are used to correct the common angularphase error and the sampling clock drift. Data rate and packet length ofthe received packet are determined using the SIGNAL field 266. Thisinformation is then used to demodulate, deinterleave, depuncture anddecode the received information symbols.

The channel compensation function estimates the frequency response ofthe channel, which is later used to compensate the effects of thechannel from the received packet, before the transmitted bits can berecovered.

One of the advantages of the OFDM communications system is that it isnot necessary to use a complicated equalizer to compensate for fadingand distortion caused by the channel. Instead of performing equalizationin the time domain, an OFDM receiver typically compensates for thechannel response in the frequency domain. This compensation usuallyrequires much less computation. The low PAPR long sync preambleactivates a variety of subcarriers (for example, 52 similar to IEEE802.11a or possibly all 64 for proprietary solutions) with a known valueduring the long sync sequence. Channel estimation and correction can bemodeled as (Equation 7):

Y _(Long,k) =H _(k) X _(Long,k) +W _(k)

where X_(Long,k) is the known k^(th) subcarrier signal of the long syncpreamble; Y_(Long,k) is the received k^(th) subcarrier signal during thelong sync preamble. H_(k) is the channel frequency response on thek^(th) subcarrier; and W_(k) is AWGN noise on the k^(th) subcarrier. Thechannel frequency response estimation on subcarrier k is given by(Equation 8):

${\hat{H}}_{k} = \frac{Y_{{Long},k}}{X_{{Long},k}}$

Since the channel impulse response is assumed to remain stationarythroughout a packet, channel estimation derived in the long syncpreamble can be used to correct the data symbols that will follow inthat same packet transmission. Such that (Equation 9):

${\hat{Y}}_{n,k} = \frac{Y_{n,k}}{H_{k}}$

where Y_(n,k) and Ŷ_(n,k) are the signals at the receiver before andafter channel correction, respectively.

Two received long sync symbols are combined and then compared to theknown transmitted long sync symbol to estimate the complex channelcoefficient for information carrying subcarriers as well as pilotsubcarriers (Equation 10).

${\hat{H}(k)} = {\frac{1}{2}{\sum\limits_{m = 0}^{1}\frac{R^{m}(k)}{S_{long}^{m}(k)}}}$

In the 64 subcarrier example, if the samples used to calculate measuredR⁰(k) and R¹(k) are consecutive in time (i.e. T₁ and T₂ from FIG. 8,respectively) then only a single reference is needed. This methodimplies that S⁰ _(long)(k) and S¹ _(long)(k) are identical. In practicalimplementations, this is not the case. Therefore, the references S⁰_(long)(k) and S¹ _(long)(k) are different.

Instead, the double guard interval (GI2) as shown FIG. 8 is effectivelysplit in half. The first 16 samples of GI2 make up a normal guardinterval, then the last 16 samples of GI2 plus the first 48 samples ofT₁ provide the 64 samples presented to the FFT for the measurement ofR⁰(k). Continuing through the long sync sequence, the last 16 samples ofT₁ make up a normal guard interval, then all 64 samples of T₂ arepresented to the FFT for the measurement of R¹(k).

A division operation is typically not used in a practical hardwareimplementation. For that reason the 1/S^(m) _(long)(k) are replaced witha multiplication by their inverse. This requires that S⁰ _(long)(k) andS¹ _(long)(k) be invertible, meaning no zero values. As an example, FIG.15 shows a 64 subcarrier low PAPR preamble in the frequency domain.

The conjugate inversion of this will make the reference for S¹_(long)(k) which is shown in FIG. 16 for the low PAPR preamble Long Sync1 reference.

There is an invertible requirement and also in an attempt to simplifythe mathematics, the formula for the channel estimate H(k) shown beforecan be modified. For example, the summation is removed and eachindividual contribution listed by itself (Equation 11).

Ĥ(k)=0.5·R ⁰(k)/S ⁰ _(long)(k)+0.5·R ¹(k)/S ¹ _(long)(k)

Replacing the references 1/S⁰ _(long)(k) with Z⁰ _(long)(k) and 1/S¹_(long)(k) with Z¹ _(long)(k), the division operations disappear. Amultiply by 0.5 is implemented as a shift right operation and that canbe left in place. The new equation is (Equation 12):

Ĥ(k)=0.5·R ⁰(k)·Z⁰ _(long)(k)+0.5·R ¹(k)·Z¹ _(long)(k)

An additional factorization of the above equation for Equation 12 movesthe 0.5 and Z¹ _(long)(k) outside the bracket (Equation 13).

Ĥ(k)=0.5·Z ¹ _(long)(k)[R ⁰(k)·Z ⁰ _(long)(k)/Z ¹ _(long)(k)+R ¹(k)]

Therefore, an alternate calculation of the reference V⁰ _(long)(k)=Z⁰_(long)(k)/Z¹ _(long)(k) which is done ahead of time reduces the formulato (Equation 13):

Ĥ(k)=0.5·Z ¹ _(long)(k)[R ⁰(k)·V ⁰ _(long)(k)+R ¹(k)]

One of the advantages of this new intermediate reference value V⁰_(long)(k) is that its representation for the low PAPR preamble is justplus or minus one (±1) values in the real or imaginary component only.This is shown in the FIG. 17. These can easily be implemented as adds orsubtracts of the specified component (real or imaginary) such as in aprocessor, FPGA or other device.

After completion of the one time per packet calculation needed for thechannel estimate, the correction can be applied for each subsequent OFDMsymbol in the transmission. By taking the samples of interest (splittingup the longer GI2) from the long sync sequence in a similar manner tothe way the data symbols will be processed, the system can guaranteethat the channel estimate is concurrent and aligned in frequency to thedata symbol frequency domain representations. This includes anysubcarrier reordering that might have been required for other aspects ofthe receiver implementation.

FIG. 18 illustrates an example receiver at 300 that takes advantage ofthe low PAPR preamble in accordance with a non-limiting example. Itreceives the OFDM communications signal that incorporates the low PAPRpreamble and can perform enhanced channel monitoring. The receiver shownin FIG. 18 shows some similarity to the receiver shown in FIG. 7 andincludes the channel estimator. The receiver 300 includes anacquisition/synchronization circuit shown at 302 that includes theinputs such as from the analog/digital converter 226 as shown in FIG. 7with a real and imaginary signal path for the A/D real signal path 303and A/D imaginary signal path 304. Each path includes a DC offsetcircuit 305, 306 and an in-phase/quadrature imbalance circuit 307, 308for both the real and imaginary values. These are input into anautomatic gain control (AGC) circuit 310 and into a timing detectcircuit 311. The timing detect circuit 311 sends the timing value notonly to the sample buffer 312 such as the data buffer 228 of FIG. 7, butalso through a coarse frequency circuit 314 and fine frequency circuit315 for the acquisition and synchronization as described before relativeto the low PAPR preamble.

After the sample buffer that corresponds such as to the data buffer 228of FIG. 7, further components receive the signal and correspond to manyof the components of FIG. 7. Guard interval removal occurs at a guardinterval removal circuit 320 followed by the FFT circuit 321 andsubcarrier demapping at a subcarrier demapper circuit 322. The channelestimate circuit 323 corresponds such as to the channel estimator 237shown in FIG. 7 and is followed by carrier phase and timing driftcorrection circuit 324 and soft bit decisions circuit 325.Deinterleaving occurs at circuit 326 followed by convolutional decodingin a convolutional decoder 327. The signal is descrambled and adescrambler circuit 328 followed by data bits output 329.

As illustrated, the channel estimate circuit is operative with a channelestimate feedback circuit 323 a that can transmit back to thetransmitter data regarding the communications channel, such as forturning OFF and ON selected subcarriers and other details as explained.This transmitter can include a processor 323 b that controls thefeedback and channel estimate calculations as well as operation of thechannel estimate circuit 323.

The low PAPR preamble is advantageous because the PAPR of the signal isabout 2.6 dB and independent of the number of subcarriers used. TypicalOFDM systems transmit known training symbols during the preamble inorder to allow the receiver to perform channel estimation. To simplifyreceiver implementation, an impulse can be transmitted over the channelin order to measure the channel's instantaneous impulse response. It iswell-known that an impulse has a large time-domain peak. The frequencydomain subcarriers will approach a time-domain impulse as thesubcarriers are added together in-phase. Preferably, the known trainingsymbols contain a sample of the channel at each subcarrier location. Ifeach of the subcarriers are added in-phase, the result is a worst-casePAPR of 10 log(N), which is essentially an impulse. In accordance with anon-limiting example of the present invention, the system produces thedesired impulse response of the channel while maintaining a low PAPR ofabout 2.6 dB, independent of the number of subcarriers.

FIG. 18A shows a high-level block diagram of a signal generator as anOFDM transmitter 340 that transmits a signal with a low PAPR preamblethat can be used for measuring the filter response of a filter 342,which can include an associated processor and measuring circuit 342 a.The processor 342 a can include an associated feedback circuit 342 b forfeeding information back to the transmitter. Alternatively, data can befed back to the signal generator as the OFDM transmitter in thisnon-limiting example.

For purposes of description, some further information on coding,interleaving, and an exemplary wireless, mobile radio communicationssystem that includes ad-hoc capability and can be modified for use isset forth. This example of a communications system that can be used andmodified in accordance with the present invention is now set forth withregard to FIGS. 19 and 20. FIG. 19 shows a number of radio devices thatcould be transmitters and receivers.

An example of a radio that could be used with such system and method isa Falcon™ III radio manufactured and sold by Harris Corporation ofMelbourne, Fla. This type of radio can support multiple wavebands form30 MHz up to 2 GHz, including L-band SATCOM and MANET. The waveforms canprovide secure IP data networking. It should be understood thatdifferent radios can be used, including software defined radios that canbe typically implemented with relatively standard processor and hardwarecomponents. One particular class of software radio is the Joint TacticalRadio (JTR), which includes relatively standard radio and processinghardware along with any appropriate waveform software modules toimplement the communication waveforms a radio will use. JTR radios alsouse operating system software that conforms with the softwarecommunications architecture (SCA) specification (seewww.jtrs.saalt.mil), which is hereby incorporated by reference in itsentirety. The SCA is an open architecture framework that specifies howhardware and software components are to interoperate so that differentmanufacturers and developers can readily integrate the respectivecomponents into a single device.

The Joint Tactical Radio System (JTRS) Software Component Architecture(SCA) defines a set of interfaces and protocols, often based on theCommon Object Request Broker Architecture (CORBA), for implementing aSoftware Defined Radio (SDR). In part, JTRS and its SCA are used with afamily of software re-programmable radios. As such, the SCA is aspecific set of rules, methods, and design criteria for implementingsoftware re-programmable digital radios.

The JTRS SCA specification is published by the JTRS Joint Program Office(JPO). The JTRS SCA has been structured to provide for portability ofapplications software between different JTRS SCA implementations,leverage commercial standards to reduce development cost, reducedevelopment time of new waveforms through the ability to reuse designmodules, and build on evolving commercial frameworks and architectures.

The JTRS SCA is not a system specification, as it is intended to beimplementation independent, but a set of rules that constrain the designof systems to achieve desired JTRS objectives. The software framework ofthe JTRS SCA defines the Operating Environment (OE) and specifies theservices and interfaces that applications use from that environment. TheSCA OE comprises a Core Framework (CF), a CORBA middleware, and anOperating System (OS) based on the Portable Operating System Interface(POSIX) with associated board support packages. The JTRS SCA alsoprovides a building block structure (defined in the API Supplement) fordefining application programming interfaces (APIs) between applicationsoftware components.

The JTRS SCA Core Framework (CF) is an architectural concept definingthe essential, “core” set of open software Interfaces and Profiles thatprovide for the deployment, management, interconnection, andintercommunication of software application components in embedded,distributed-computing communication systems. Interfaces may be definedin the JTRS SCA Specification. However, developers may implement some ofthem, some may be implemented by non-core applications (i.e., waveforms,etc.), and some may be implemented by hardware device providers.

For purposes of description only, a brief description of an example of acommunications system that includes communications devices thatincorporate the phase and Doppler (frequency) tracking, in accordancewith a non-limiting example, is described relative to a non-limitingexample shown in FIG. 19. This high-level block diagram of acommunications system includes a base station segment and wirelessmessage terminals that could be modified for use with the presentinvention. The base station segment includes a VHF radio 360 and HFradio 362 that communicate and transmit voice or data over a wirelesslink to a VHF net 364 or HF net 366, each which include a number ofrespective VHF radios 368 and HF radios 370, and personal computerworkstations 372 connected to the radios 368,370. Ad-hoc communicationnetworks 373 are interoperative with the various components asillustrated. The entire network can be ad-hoc and include source,destination and neighboring mobile nodes. Thus, it should be understoodthat the HF or VHF networks include HF and VHF net segments that areinfrastructure-less and operative as the ad-hoc communications network.Although UHF and higher frequency radios and net segments are notillustrated, these are included for DAMA and satellite operation.

The radio can include a demodulator circuit 362 a and appropriateconvolutional encoder circuit 362 b, block interleaver 362 c, datarandomizer circuit 362 d, data and framing circuit 362 e, modulationcircuit 362 f, matched filter circuit 362 g, block or symbol equalizercircuit 362 h with an appropriate clamping device, deinterleaver anddecoder circuit 362 i modem 362 j, and power adaptation circuit 362 k asnon-limiting examples. A vocoder circuit 362 l can incorporate thedecode and encode functions and a conversion unit could be a combinationof the various circuits as described or a separate circuit. A clockcircuit 362 m can establish the physical clock time and through secondorder calculations as described below, a virtual clock time. The networkcan have an overall network clock time. These and other circuits operateto perform any functions necessary for the present invention, as well asother functions suggested by those skilled in the art. Other illustratedradios, including all VHF (or UHF) and higher frequency mobile radiosand transmitting and receiving stations can have similar functionalcircuits. Radios could range from 30 MHz to about 2 GHz as non-limitingexamples.

The base station segment includes a landline connection to a publicswitched telephone network (PSTN) 380, which connects to a PABX 382. Asatellite interface 384, such as a satellite ground station, connects tothe PABX 382, which connects to processors forming wireless gateways 386a, 386 b. These interconnect to the UHF, VHF radio 360 or HF radio 362,respectively. The processors are connected through a local area networkto the PABX 382 and e-mail clients 390. The radios include appropriatesignal generators and modulators.

An Ethernet/TCP-IP local area network could operate as a “radio” mailserver. E-mail messages could be sent over radio links and local airnetworks using STANAG-5066 as second-generation protocols/waveforms, thedisclosure which is hereby incorporated by reference in its entiretyand, of course, preferably with the third-generation interoperabilitystandard: STANAG-4538, the disclosure which is hereby incorporated byreference in its entirety. An interoperability standard FED-STD-1052,the disclosure which is hereby incorporated by reference in itsentirety, could be used with legacy wireless devices. Examples ofequipment that can be used in the present invention include differentwireless gateway and radios manufactured by Harris Corporation ofMelbourne, Fla. This equipment could include RF5800, 5022, 7210, 5710,5285 and PRC 117 and 138 series equipment and devices as non-limitingexamples.

These systems can be operable with RF-5710A high-frequency (HF) modemsand with the NATO standard known as STANAG 4539, the disclosure which ishereby incorporated by reference in its entirety, which provides fortransmission of long distance radio at rates up to 9,600 bps. Inaddition to modem technology, those systems can use wireless emailproducts that use a suite of data-link protocols designed and perfectedfor stressed tactical channels, such as the STANAG 4538 or STANAG 5066,the disclosures which are hereby incorporated by reference in theirentirety. It is also possible to use a fixed, non-adaptive data rate ashigh as 19,200 bps with a radio set to ISB mode and an HF modem set to afixed data rate. It is possible to use code combining techniques andARQ.

A communications system that incorporates communications devices can beused in accordance with non-limiting examples of the present inventionand is shown in FIG. 20. A transmitter is shown at 391 and includesbasic functional circuit components or modules, including a forwarderror correction encoder 392 a that includes a puncturing module, whichcould be integral to the encoder or a separate module. The decoder 392 aand its puncturing module includes a function for repeating as will beexplained below. Encoded data is interleaved at an interleaver 392 b,for example, a block interleaver, and in many cases modulated atmodulator 392 c. This modulator can map the communications data intodifferent symbols based on a specific mapping algorithm to form acommunications signal. For example, it could form Minimum Shift Keyingor Gaussian Minimum Shift Keying (MSK or GMSK) symbols. Other types ofmodulation could be used in accordance with non-limiting examples of thepresent invention. Up-conversion and filtering occurs at an up-converterand filter 392 d, which could be formed as an integrated module orseparate modules. Communications signals are transmitted, for example,wirelessly to receiver 393.

At the receiver 393, down conversion and filtering occurs at a downconverter and filter 394 a, which could be integrated or separatemodules. The signal is demodulated at demodulator 394 b anddeinterleaved at deinterleaver 394 c. The deinterleaved data (i.e., bitsoft decisions) is decoded and depunctured (for punctured codes),combined (for repeated codes) and passed through (for standard codes) atdecoder 394 d, which could include a separate or integrated depuncturingmodule. The system, apparatus and method can use different modules anddifferent functions. These components as described could typically becontained within one transceiver.

It should be understood, in one non-limiting aspect of the presentinvention, a rate 1/2, K=7 convolutional code can be used as an industrystandard code for forward error correction (FEC) during encoding. Forpurposes of understanding, a more detailed description of basiccomponents now follows. A convolutional code is an error-correctingcode, and usually has three parameters (n, k, m) with n equal to thenumber of output bits, k equal to the number of input bits, and m equalto the number of memory registers, in one non-limiting example Thequantity k/n could be called the code rate with this definition and is ameasure of the efficiency of the code. K and n parameters can range from1 to 8, m can range from 2 to 10, and the code rate can range from 1/8to 7/8 in non-limiting examples. Sometimes convolutional code chips arespecified by parameters (n, k, L) with L equal to the constraint lengthof the code as L=k (m−1). Thus, the constraint length can represent thenumber of bits in an encoder memory that would affect the generation ofn output bits. Sometimes the letters may be switched depending on thedefinitions used.

The transformation of the encoded data is a function of the informationsymbols and the constraint length of the code. Single bit input codescan produce punctured codes that give different code rates. For example,when a rate 1/2 code is used, the transmission of a subset of the outputbits of the encoder can convert the rate 1/2 code into a rate 2/3 code.Thus, one hardware circuit or module can produce codes of differentrates. Punctured codes allow rates to be changed dynamically throughsoftware or hardware depending on channel conditions, such as rain orother channel impairing conditions.

An encoder for a convolutional code typically uses a look-up table forencoding, which usually includes an input bit as well as a number ofprevious input bits (known as the state of the encoder), the table valuebeing the output bit or bits of the encoder. It is possible to view theencoder function as a state diagram, a tree diagram or a trellisdiagram.

Decoding systems for convolutional codes can use 1) sequential decoding,or 2) maximum likelihood decoding, also referred to as Viterbi decoding,which typically is more desirable. Sequential decoding allows bothforward and backward movement through the trellis. Viterbi decoding asmaximum likelihood decoding examines a receive sequence of given length,computes a metric for each path, and makes a decision based on themetric.

Puncturing convolutional codes is a common practice in some systems andis used in accordance with non-limiting examples of the presentinvention. It should be understood that in some examples a puncturedconvolutional code is a higher rate code obtained by the periodicelimination of specific code bits from the output of a low rate encoder.Punctured convolutional code performance can be degraded compared withoriginal codes, but typically the coding rate increases.

Some of the basic components that could be used as non-limiting examplesof the present invention include a transmitter that incorporates aconvolutional encoder, which encodes a sequence of binary input vectorsto produce the sequence of binary output vectors and can be definedusing a trellis structure. An interleaver, for example, a blockinterleaver, can permute the bits of the output vectors. The interleaveddata would also be modulated at the transmitter (by mapping to transmitsymbols) and transmitted. At a receiver, a demodulator demodulates thesignal.

A block deinterleaver recovers the bits that were interleaved. A Viterbidecoder could decode the deinterleaved bit soft decisions to producebinary output data.

Often a Viterbi forward error correction module or core is used thatwould include a convolutional encoder and Viterbi decoder as part of aradio transceiver as described above. For example if the constraintlength of the convolutional code is 7, the encoder and Viterbi decodercould support selectable code rates of 1/2, 2/3, 3/4, 4/5, 5/6, 6/7, 7/8using industry standard puncturing algorithms,

Different design and block systems parameters could include theconstraint length as a number of input bits over which the convolutionalcode is computed, and a convolutional code rate as the ratio of theinput to output bits for the convolutional encoder. The puncturing ratecould include a ratio of input to output bits for the convolutionalencoder using the puncturing process, for example, derived from a rate1/2 code.

The Viterbi decoder parameters could include the convolutional code rateas a ratio of input to output bits for the convolutional encoder. Thepuncture rate could be the ratio of input to output bits for theconvolutional encoder using a puncturing process and can be derived froma rate 1/2 mother code. The input bits could be the number of processingbits for the decoder. The Viterbi input width could be the width ofinput data (i.e. soft decisions) to the Viterbi decoder. A metricregister length could be the width of registers storing the metrics. Atrace back depth could be the length of path required by the Viterbidecoder to compute the most likely decoded bit value. The size of thememory storing the path metrics information for the decoding processcould be the memory size. In some instances, a Viterbi decoder couldinclude a First-In/First-Out (FIFO) buffer between depuncture andViterbi function blocks or modules. The Viterbi output width could bethe width of input data to the Viterbi decoder.

The encoder could include a puncturing block circuit or module as notedabove. Usually a convolutional encoder may have a constraint length of 7and take the form of a shift register with a number of elements, forexample, 6. One bit can be input for each clock cycle. Thus, the outputbits could be defined by a combination of shift register elements usinga standard generator code and be concatenated to form an encoded outputsequence. There could be a serial or parallel byte data interface at theinput. The output width could be programmable depending on the puncturedcode rate of the application.

A Viterbi decoder in non-limiting examples could divide the input datastream into blocks, and estimate the most likely data sequence. Eachdecoded data sequence could be output in a burst. The input andcalculations can be continuous and require four clock cycles for everytwo bits of data in one non-limiting example. An input FIFO can bedependent on a depuncture input data rate.

It should also be understood that the radio device is not limited toconvolutional codes and similar FEC, but also turbo codes could be usedas high-performance error correction codes or low-density parity-checkcodes that approach the Shannon limit as the theoretical limit ofmaximum information transfer rate over a noisy channel. Thus, someavailable bandwidth can be increased without increasing the power of thetransmission. Instead of producing binary digits from the signal, thefront-end of the decoder could be designed to produce a likelihoodmeasure for each bit.

The system, in accordance with non-limiting examples of the presentinvention, can be used in multiprocessor embedded systems and relatedmethods and also used for any type of radio software communicationsarchitecture as used on mainframe computers or small computers,including laptops with an added transceiver, such as used by militaryand civilian applications, or in a portable wireless communicationsdevice 420 as illustrated in FIG. 21. The portable wirelesscommunications device is illustrated as a radio that can include atransceiver as an internal component and handheld housing 422 with anantenna 424 and control knobs 426. A Liquid Crystal Display (LCD) orsimilar display can be positioned on the housing in an appropriatelocation for display. The various internal components, including dualprocessor systems for red and black subsystems and software that isconforming with SCA, is operative with the illustrated radio. Although aportable or handheld radio is disclosed, the architecture as describedcan be used with any processor system operative with the system inaccordance with non-limiting examples of the present invention. Anexample of a communications device that could incorporate thecommunications system in accordance with non-limiting examples of thepresent invention, is the Falcon® III manpack or tactical radio platformmanufactured by Harris Corporation of Melbourne, Fla.

This application is related to copending patent applications entitled,“LOW PEAK-TO-AVERAGE POWER RATIO (PAPR) PREAMBLE FOR ORTHOGONALFREQUENCY DIVISION MULTIPLEXING (OFDM) COMMUNICATIONS” and “ORTHOGONALFREQUENCY DIVISION MULTIPLEXING (OFDM) COMMUNICATIONS DEVICE AND METHODTHAT INCORPORATES LOW PAPR PREAMBLE AND RECEIVER CHANNEL ESTIMATECIRCUIT,” and “ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM)COMMUNICATIONS DEVICE AND METHOD THAT INCORPORATES LOW PAPR PREAMBLE ANDFREQUENCY HOPPING,” and “ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM) COMMUNICATIONS DEVICE AND METHOD THAT INCORPORATES LOW PAPRPREAMBLE WITH CIRCUIT FOR MEASURING FREQUENCY RESPONSE OF THECOMMUNICATIONS CHANNEL,” which are filed on the same date and by thesame assignee and inventors, the disclosures which are herebyincorporated by reference.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A communications device, comprising: a source of communications dataand a training sequence corresponding to a preamble; a modulation andmapping circuit that modulates the communications data and trainingsequence into a plurality of variable subcarriers that are orthogonal toeach other to form an Orthogonal Frequency Division Multiplexing (OFDM)communications signal having modulated subcarriers carrying thecommunications data forming a data payload and modulated subcarriersforming the preamble; a circuit for turning OFF and ON selectedsubcarriers based on an encryption algorithm to increase transmit powerand signal-to-noise ratio per subcarrier and reduce the Inter-CarrierInterference (ICI); and wherein said modulation and mapping circuitapplies a quadratic modulation to the subcarriers carrying the trainingsequences to produce a low peak-to-average power ratio (PAPR) preamblewith about a PAPR value of 2.6 decibels (dB).
 2. The communicationsdevice according to claim 1, and further comprising a circuit forspreading the selected ON subcarriers over the frequency domain toreduce average power for enhanced Low Probability of Interception/LowProbability of Detection (LPI/LPD) and allow for more transmit powerwithin a spectral mask and reduce adverse effects of Frequency SelectiveFading.
 3. The communications device according to claim 1, and furthercomprising a pseudo-random sequence generator operative with themodulation and mapping circuit for generating pseudo-random signals tothe modulation and mapping circuit based on an encryption algorithm forfrequency hopping each subcarrier at an OFDM symbol rate.
 4. Thecommunications device according to claim 1, wherein said modulation andmapping circuit is operative for independently amplitude modulating eachsubcarrier carrying the data sequences.
 5. The communications deviceaccording to claim 1, wherein the low PAPR preamble is independent ofthe number of subcarriers forming the OFDM communications signal.
 6. Thecommunications device according to claim 1, wherein said modulation andmapping circuit is operative for setting quadratic phases of sinusoidsforming the subcarriers carrying the training sequences.
 7. Thecommunications device according to claim 6, wherein said modulation andmapping circuit is operative for setting the phases of the sinusoids toa specific angle.
 8. The communications device according to claim 1,wherein the preamble includes a long sync sequence for channel and finefrequency offset estimation having a guard interval (GI).
 9. Thecommunications device according to claim 1, wherein said modulation andmapping circuit is operative for forming each of the subcarriers thatcarries the training sequences of the OFDM communications signal to haveequal amplitude.
 10. A method of communicating data, comprising:receiving communications data to be transmitted and a training sequencecorresponding to a preamble; generating an Orthogonal Frequency DivisionMultiplexing (OFDM) communications signal having modulated subcarrierscarrying the communications data forming a data payload and subcarrierscarrying the training sequences forming the preamble; turning OFF and ONselected subcarriers based on an encryption algorithm to increasetransmit power and signal-to-noise ratio per subcarrier and reduce theInter-Carrier Interference (ICI); applying quadratic phase modulation tosubcarriers carrying the training sequences to produce a lowpeak-to-average power ratio (PAPR) preamble with about a PAPR value of2.6 decibels (dB); and transmitting the OFDM communications signal alonga communications channel to a receiver.
 11. The method according toclaim 10, which further comprises spreading the selected ON subcarriersover the frequency domain to reduce average power for enhanced LowProbability of Interception/Low Probability of Detection (LPI/LPD) andallow for more transmit power within a spectral mask and reduce adverseeffects of Frequency Selective Fading.
 12. The method according to claim10, which further comprises generating pseudo-random signals to themodulation and mapping circuit based on an encryption algorithm forfrequency hopping each subcarrier at an OFDM symbol rate.
 13. The methodaccording to claim 10, which further comprises equally spacing infrequency each of the subcarriers carrying the training sequences. 14.The method according to claim 10, which further comprises forming eachof the subcarriers carrying the training sequences to have equalamplitude.
 15. The method according to claim 10, which further comprisesindependently amplitude modulating each subcarrier carrying the datasequences.
 16. The method according to claim 10, wherein the low PAPRpreamble is independent of the number of subcarriers forming the OFDMcommunications signal.
 17. The method according to claim 10, whichfurther comprises measuring fluctuations within a flat-topped spectrumof the OFDM communications signal corresponding to the preamble toreflect the frequency response of the communications channel.
 18. Themethod according to claim 10, which further comprises setting quadraticphases of sinusoids forming the subcarriers carrying the trainingsequences.
 19. The method according to claim 18, which further comprisessetting the phases of the sinusoids to a specific angle.
 20. A method ofcommunicating data, comprising: receiving communications data to betransmitted and a training sequence corresponding to a preamble;generating an Orthogonal Frequency Division Multiplexing (OFDM)communications signal having modulated subcarriers carrying thecommunications data forming a data payload and subcarriers carryingtraining sequences forming the preamble, wherein the preamble includes along sync sequence for channel and fine frequency offset estimationhaving a guard interval (GI); turning OFF and ON selected subcarriersbased on an encryption algorithm to increase transmit power andsignal-to-noise ratio and reduce the Intercarrier Interference (ICI);applying a quadratic phase modulation to subcarriers carrying thetraining sequences to produce a low peak-to-average power ratio (PAPR)preamble with about a PAPR value of 2.6 decibels (dB); and splitting theguard interval from the long sync sequence and processing into valuesthat represent the inverse low PAPR preamble as plus or minus one (±1)values in a real or imaginary component as adds and subtracts forchannel estimation.
 21. The method according to claim 20, which furthercomprises spreading the selected ON subcarriers over the frequencydomain to reduce average power for enhanced Low Probability ofInterception/Low Probability of Detection (LPI/LPD) and allow for moretransmit power within a spectral mask and reduce adverse effects ofFrequency Selective Fading.
 22. The method according to claim 20, whichfurther comprises generating pseudo-random signals to the modulation andmapping circuit based on an encryption algorithm for frequency hoppingeach subcarrier at an OFDM symbol rate.
 23. The method according toclaim 20, which further comprises using the “n” samples of the guardinterval as a normal GI and portions of the long sync sequence for FastFourier Transform (FFT) processing for channel estimation.